1. Field of the Invention
This invention relates to a method of suppressing the influence of in-fiber non-linear distortion which is undesirable in to a long-distance coherent optical communication system employing light amplifier repeaters.
2. Description of the Related Art
Optical fiber amplifier, allow the possibility of long-distance multi-stage repeatered transmission optical communication systems, several thousand km long. Coherent optical communication is considered to be an advantageous system for a long-distance transmission system. The reason is that the coherent system has advantages in that it has a high receiver sensitivity and also can compensate simply for the influence of waveform dispersion of a transmission line in an intermediate frequency (IF) band of a receiver.
However, when the output power of an optical repeater is high, distortion by a non-linear optical effect is sometimes produced in an optical fiber, which limits the transmission distance. This phenomenon is particularly prominent when tile dispersion value of an optical fiber is high as described below. When a high waveform dispersion is present in a transmission line, amplitude distortion is produced even in signal light having a fixed envelope such as frequency-shift-keying (FSK) modulated light during transmission. The amplitude distortion is converted into phase distortion of the signal light by way of the Kerr effect of the fiber. Consequently, the phase variation (frequency shift amount) of the signal light after transmission deviates from a value set in advance on the transmission side, and accordingly, the receiver sensitivity deteriorates. Since this phenomenon increases as the output power of the repeater increases, it is a serious obstacle to elongation of the repeater distance.